1. Field of the Invention
The present invention relates to a fuel supply apparatus for an internal combustion engine, and more particularly to a fuel supply apparatus for an internal combustion engine having a first fuel supply system and a second fuel supply system for supplying a fuel to an internal combustion engine.
2. Description of the Background Art
Examples of a method of supplying a fuel to an internal combustion engine such as a gasoline engine and a diesel engine incorporated in a vehicle such as a truck or the like include in-cylinder injection for directly injecting the fuel into a cylinder of the internal combustion engine, manifold injection for injecting the fuel into an intake manifold for taking air into the cylinder, and a combination thereof, that is, in-cylinder injection/manifold injection switching between in-cylinder injection and manifold injection in accordance with an operation state of the internal combustion engine.
For example, Japanese Patent Laying-Open No. 07-103048 discloses a fuel injection apparatus for an internal combustion engine carrying out in-cylinder injection/manifold injection in accordance with an operation state of the internal combustion engine. The fuel injection apparatus for the internal combustion engine includes a fuel supply apparatus, a first fuel injection mechanism having a manifold injector performing manifold injection (a fuel injection valve for injection into engine intake manifold), and a second fuel injection mechanism having an in-cylinder injector performing in-cylinder injection (a fuel injection valve for injection into cylinder). The fuel supply apparatus has a first fuel supply system that applies pressure to the fuel within a fuel tank by a low-pressure pump and supplies the fuel to the first fuel injection mechanism, and a second fuel supply system that further applies pressure to the fuel pressurized by the low-pressure pump by a high-pressure pump and supplies the resultant fuel to the second fuel injection mechanism. This fuel injection apparatus for the internal combustion engine controls injection of the first and second fuel injection mechanisms in accordance with a map prepared based on a fuel supply quantity (fuel injection quantity) and degree of accelerator press-down (an amount of press-down of an accelerator pedal). Specifically, the map is divided into an injection region where solely the first fuel injection mechanism is used, an injection region where both of the first and second fuel injection mechanisms are used, and an injection region where solely the second fuel injection mechanism is used. Then, control means controls the injection of the first and second fuel injection mechanisms in accordance with the operation state of the internal combustion engine.
In the conventional fuel injection apparatus for an internal combustion engine as described above, a high-pressure pump is provided for supplying a high-pressure fuel to the second fuel injection mechanism. The high-pressure pump applies pressure to the fuel in the following manner. A cam rotates in accordance with rotation of a crankshaft of the internal combustion engine. This causes reciprocating motion of a plunger of the high-pressure pump, so that further pressure is applied to the fuel having been pressurized by the low-pressure pump. The high-pressure pump continues to operate by rotation of the crankshaft of the internal combustion engine, even in the state where the control means controls not to inject the fuel from the in-cylinder injector, i.e., even in the state where the second fuel injection mechanism is not actuated. Consequently, pulsation occurs when the fuel is suctioned from the first fuel supply system by the high-pressure pump or when excessive fuel is returned. Such pulsation causes fluctuation in pressure of the fuel, i.e., fuel pressure, within the pipe of the first fuel supply system, and the fluctuation of the fuel pressure propagates to the first fuel injection mechanism. Although the control means controls injection timing and quantity of the fuel injected from the first fuel injection mechanism into the intake manifold of the internal combustion engine, the first fuel injection mechanism has not been able to inject the fuel of an injection supply quantity that is determined based on the operation state of the internal combustion engine, due to propagation of pulsation to the first fuel injection mechanism. When the fuel of a prescribed fuel supply quantity that should be supplied to the internal combustion engine cannot be supplied from the fuel injection apparatus, there will occur fluctuation of air-fuel ratio, leading to degradation in fuel efficiency and driving properties.